1. Field of the Invention
The present invention relates to an image forming apparatus including a developing device of the type causing a developer to form a magnet brush on the surface of a developer carrier in a developing region, or nip for development, and contact and thereby develops a latent image formed on an image carrier.
2. Description of the Background Art
It is a common practice with a copier, printer, facsimile apparatus or similar electrophotographic or electrostatic image forming apparatus to electrostatically form a latent image on an image carrier in accordance with image data. The image carrier may be implemented by a photoconductive drum or a photoconductive belt. A developing device develops the latent image with toner and thereby produces a corresponding toner image. A current trend in the imaging art is toward a magnet brush type developing system using a toner and carrier mixture or two-ingredient type developer. This type of developing system is desirable from the standpoint of image transfer, halftone reproducibility, and stability of development against varying temperature and humidity. Specifically, a developing device using this type of system causes the developer to rise in the form of brush chains on a developer carrier, so that toner contained in the developer is transferred to a latent image formed on the image carrier at a developing region. The developing region refers to a range over which a magnet brush rises on the developer carrier and contacts the image carrier.
The developer carrier is generally made up of a hollow cylindrical sleeve or developing sleeve and a magnet roller surrounded by the sleeve. The magnet roller forms a magnetic field for causing the developer deposited on the sleeve to rise in the form of a magnet brush. When the developer rises on the sleeve, carrier particles contained therein rise along magnetic lines of force generated by the magnet roller. Charged toner particles are deposited on each of such carrier particles. The magnet roller has a plurality of magnetic poles formed by rod-like magnets and including a main pole for causing the developer to rise in the developing region.
In the above-described configuration, when at least one of the sleeve and magnet roller moves, it conveys the developer to the developing region. In the developing region, the developer rises in the form of brush chains along the magnetic lines of force generated by the main pole. The brush chains or heads contact the surface of the image carrier while yielding themselves. While the brush chains sequentially rub themselves against a latent image formed on the image carrier on the basis of a difference in linear velocity between the developer carrier and the image carrier, the toner is transferred from the developer carrier to the image carrier.
In a developing device of the type described, the flux density of the main pole in the normal direction decreases little by little toward opposite ends of the developing region while the flux density in the tangential direction increases little by little. As a result, the magnet brush tilts more at the end portions than at the intermediate portion of the developing region, resulting in defective images. For example, the crossing portions of solid lines, a black solid image or a halftone solid image is lost at its trailing edge portion (local omission hereinafter). Further, horizontal lines and dots are not faithfully reproduced. More specifically, horizontal lines included in a lattice pattern having the same width are rendered thinner than vertical lines or a dot image is not developed at all.
Japanese Patent Application No. 2000-296371, for example, discloses an image forming apparatus constructed to implement desirable image density and image quality by obviating the above-mentioned defects. The apparatus taught in this document uses a magnet formed of ion-neodymium-boron alloy, iron-neodymium-boron alloy bond or similar rare earth metal alloy or samarium alloy in order to reduce a half width while maintaining a magnetic force required of the main pole. Such a magnet, however, noticeably increases the cost of the magnet roller. This problem is particularly serious when it comes to a color image forming apparatus.
Japanese Patent No. 2,773,151, for example, proposes to position the peak of the variation of a magnetic field component (flux density) in the tangential direction in the developing region and to limit the absolute value of the peak to 30 gauss/degree. With such a peak, according to the above document, it is possible to cause the carrier to sufficiently fall down at the opposite sides of the developing region.
The above Japanese Patent describes that as for the flux density of a horizontal magnetic field component, the illustrative embodiment stabilizes the variation ratio of the density around the center, where the flux density is minimum, more than the conventional device, and increases the variation ratio at a preselected distance from the center at both sides of the center. The document further describes that the vertical and horizontal magnetic field components each vary by a great ratio at opposite end portions of the developing region, and therefore the rise/fall of the magnet brush at the opposite end portions is sharp. Theoretically, if the developing region is relatively broad, it may be possible to form portions where the magnetic force density component noticeably varies at opposite ends of the developing region while stabilizing the variation of the magnetic force density at the intermediate portion. In practice, however, the developing region available with an image forming apparatus of the type using a toner and carrier mixture is so narrow, it is difficult to locate the peak of the flux density in the tangential direction at opposite ends of the developing region. Moreover, a decrease in the diameter of the sleeve results in a decrease in the distance for an angle of 1 degree on the surface of the sleeve, so that the fall-down of the carrier particles has little effect.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2000-305360.
It is a fist object of the present invention to provide a cost effective, image forming apparatus capable of increasing image density and faithfully reproducing even low-contrast images.
It is a second object of the present invention to provide an image forming apparatus capable of reducing image defects, including granularity and local omission, to thereby enhance image quality even if a range over which a magnet brush and an image carrier contact is reduced.
In accordance with the present invention, in a developing device including a plurality of developing sections, each developing section includes a developer carrier that causes a developer deposited thereon to form a magnet brush and contact an image carrier. The developer carrier includes a rotatable nonmagnetic sleeve and a stationary magnet roller accommodated in said sleeve. The magnet roller has a magnetic pole for scooping up the developer to the sleeve, a magnetic pole for conveying the developer deposited on the sleeve, and a main magnetic pole for causing the developer to rise on the sleeve in the form of the magnet brush. The developing sections each include at least one developing section in which the flux density of the main magnetic pole in the normal direction has an attenuation ratio of 40% or above and at least one developing section in which the flux density has an attenuation ratio of 30% or below.
Also, in accordance with the present invention, in a developing device including a plurality of developing sections, each developing section includes a developer carrier that causes a developer deposited thereon to form a magnet brush and contact an image carrier. The developer carrier includes a rotatable nonmagnetic sleeve and a stationary magnet roller accommodated in said sleeve. The magnet roller has a magnetic pole for scooping up the developer to the sleeve, a magnetic pole for conveying the developer deposited on the sleeve, and a main magnetic pole for causing the developer to rise on the sleeve in the form of the magnet brush. The developing sections each include at least one developing section in which the main pole has a half width of 22xc2x0 or below and at least one developing section in which the half value is 25xc2x0 or above.